Brain metastasis is the most ominous form of relapse from carcinomas and melanoma. Its annual incidence nears 200,000 cases in the U.S, and is almost invariably lethal. Yet, brain metastasis suffers from a dearth of experimental models, mechanistic insights, therapeutic targets, and research in general. The biology of brain metastatic cells is deeply intertwined with the singular features of the brain microenvironment and, in particular, the perivascular niche. After extravasation through the blood-brain barrier (BBB), which requires a set of specialized functions, cancer cells adhere to and stretch over the basal side of the capillary endothelium (vascular cooption), attract and interact with astrocytes, and grow within the perivascular niche. Brain endothelia and astrocytes react by releasing factors that cancer cells may employ to their advantage. As the tumor grows it remodels the extant vasculature, forming capillary loops embedded in the growing nodule. Radiation therapy preceded or not by surgery, is the standard of care for overt brain metastasis in patients. Relapse after RT is typical, however, raising many questions about the mechanisms that support the viability of residual disease in the irradiated tissue. In Project 2, the Massagu6 lab is undertaking a multidisciplinary approach based on human tissue and data sets, mouse models, and molecular genetic dissection to address the problem of brain metastasis. The two main sources of cerebral metastasis are lung adenocarcinoma and breast adenocarcinoma, and as such these two modalities are the subject of our study. We will leverage a recently identified set of candidate brain metastasis genes (Bos et al Nature 2009; Nguyen et al Cell 2009), and experimental approaches that we have developed for the mechanistic delineation of metastasis to brain, bone (Kang et al Cancer Cell 2003; Zhang et al Cancer Cell 2009) and lung (Minn et al Nature 2005; Gupta et al Nature 2007; Tavazoie Nature 2008; Padua et al Celt 2008). We will investigate, in Aim 1, the functional role of candidate brain metastasis genes that we recently identified whose expression is linked to pro-metastatic interactions of cancer cells with the blood-brain barrier, the abluminal endothelial surface, and astrocytes, microglia and neurons in the perivascular niche; in Aim 2 and Aim 3, the genes and pathways that become activated in situ, in brain endothelial cells, astrocytes and other cell types as a reaction to infiltrating cancer cells, or in cancer cells on exposure to the brain microenvironment; in Aim 4, the role of these microenvironment signals as pro-metastatic mediators, as assessed by genetic means as well as with preclinical agents; and, in Aim 5, the identity of genes and pathways that are specifically activated in cancer cells that resist radiation therapy . Multiple levels of interdependency exist between our specific Aims 2 to 5 and Project 1 (Holland), and between Aims 3-4 and Project 3 (Rafii). A Pathology Core will be pivotal for human tissue procurement and histopathological analysis, a trademark of our commitment to providing clinical validation of experimentally identified mediators of metastasis.